Their entire body is covered with the bodywall - an exoskeleton. It is made of (a) cuticle (b) epidermis and (c) dermis. (a) cuticle - This is a non-cellular layer. It contains lipids (fats) and proteins. It is hardened with chitin and may be further strengthened with salts of Calcium and sclerotin. Sclerotin is hard and is formed by tanning process from arthropodin. The cuticle may have color giving pigments. The cuticle prevents damage to the animal bodies from desiccation, mechanical injuries and invasion by microorganisms. (b) epidermis – The epidermis is a live layer with many different types of cells, such as - Trichogen cells forming hard parts of sensory hairs, nerve cells which supply the mechanoreceptors, glial cells enveloping nerve cells, glandular cells - secreting pheromones and wax. The epidermis is thin secretes cuticle and helps in maintaining the cuticle. The epidermal cells synthesize and secrete chitin, the second most abundant organic substance on the surface earth. (Want to know which the most abundant? Refer please – Skin the wonderwrap-10) (c) dermis - The dermis is also a live layer with loose connective tissue. Dermal glands secerte cuticulin of the epicuticle.
The arthropods ventured onto the land away from their ancestral, aquatic home. To prevent drying in new climatic conditions their body developed a hard cover. A very hard, stiff, and thick body wall would conserve water and give protection but would make movement and location impossible. A balance has been struck by splitting the body into of distinct segments and covering the segments by plates of skeletal matter. Thus giving mechanical protection of an armor and retaining flexibility at the same time. Even a mature cuticle is highly flexible on account of a special protein called resilin.
Resilin is rubber like and has many amino acid chains stuck to each other and running randomly in all directions.
The plates on the back are called terga and those on belly side are called sterna.
Whether on the back i.e. dorsal side or on the side of belly i.e. on ventral side the plates are called sclerites. The sclerites at their broad edges(sutures) slide onto each other to some extent allowing a little telescoping of segments.
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